fix(csharp): preserve constructor param name when property is renamed to avoid class-name clash

[joelle-a-dev]

Summary

Fixes #12757.

When a C# model property collides with its own class name (e.g. error_message on class ErrorMessage), AbstractCSharpCodegen.postProcessModels renames the property with a leading underscore (_ErrorMessage) to avoid a compile error.

The generated constructor's parameter name, however, was derived at template-render time by camelizing that already-renamed property name via the lambda.camelcase_param mustache lambda. DefaultCodegen.camelize() strips leading underscores as part of its snake_case-to-camelCase conversion, so the parameter ended up as errorMessage — identical to what it would have been without the rename — instead of _errorMessage.

Since Newtonsoft.Json matches constructor parameters to properties by name (case-insensitively, using the C# member name) when deserializing via a non-default constructor, the mismatched parameter name (errorMessage vs. property _ErrorMessage) meant the argument was never bound, silently leaving the property null (and throwing on required-property validation).

Fix

• AbstractCSharpCodegen.postProcessModels now computes the constructor parameter name via toParamName(var.name) before the class-name-conflict rename, and when the conflict is detected, prepends _ to both the property name and the precomputed parameter name in lockstep (rather than letting the template re-derive it from the mutated name). The result is stored as vendorExtensions["x-parameter-name"].
• modelGeneric.mustache now reads {{vendorExtensions.x-parameter-name}} instead of re-camelizing {{name}} via lambda.camelcase_param at every constructor call site.

This only affects the csharp generator's own templates; csharp-dotnet2, aspnetcore, and nancyfx (the other AbstractCSharpCodegen subclasses) use separate template directories and are unaffected.

Test plan

• ./mvnw -pl modules/swagger-codegen -am compile succeeds.
• Built the CLI jar and regenerated C# code from the exact spec in [C#] Properties are escaped with _, which causes deserialization to fail #12757 — the constructor now reads public ErrorMessage(string errorId, string _errorMessage) and assigns this._ErrorMessage = _errorMessage;, so the parameter now matches the property.
• Existing test suites (csharp, csharpdotnettwo, aspnetcore, nancyfx, CamelCaseLambdaTest) pass: 34 tests, 0 failures.